In order to store or transmit a digital image with efficiency, it is necessary to compressively encode the image signal. As a typical method for compressively encoding a digital image signal with efficiency, there is discrete cosine transformation (DCT) which is a main technique in typical standards, JPEG (Joint Photographic Experts Group) and MPEG (Moving Picture Experts Group). Besides the DCT, there are waveform coding methods such as sub-band coding, wavelet coding, and fractal coding. Further, in order to eliminate a redundant signal between images, inter-image prediction using motion compensation is performed to obtain a difference between images, i.e., a difference between an image currently being processed and a reference image, and this difference signal is subjected to waveform-coding, whereby coded data of a high compression ratio is obtained.
In recent years, a method of compressive coding has been proposed, in which plural objects constituting an image are individually encoded and transmitted so that reproduction of the image can be performed in object units. When the image so encoded is reproduced, the respective objects are decoded individually, and the decoded objects are synthesized to display the image.
When images are synthesized, required is information showing, pixel by pixel, whether the background is hidden by the overlaying image or not. This information used for synthesis is called "significant signal", and a pixel in which the background is hidden is called "significant pixel".
Further, edition of a moving picture is facilitated by coding an image signal in object units and combining the objects as desired using the coded signal. Furthermore, depending on busyness of transmission line, performance of reproduction apparatus, and tastes of viewer, it is possible to see a moving picture without reproducing relatively unimportant objects.
Furthermore, when an image (object) having an arbitrary shape is encoded, a coding method adaptive to the shape, for example, shape adaptive DCT, is employed. Alternatively, an insignificant region of the image (a region outside the object and comprising insignificant pixels only) is padded using a prescribed method and then encoded using conventional 8.times.8 cosine transformation or the like.
On the other hand, in a prediction region (e.g., a block comprising 16.times.16 pixels) which is obtained by motion-compensating a reference image reproduced in the past to eliminate a redundant signal between images, insignificant pixels may be included at the boundary of the object. For such a prediction region, it is padded first and then a difference between the prediction region and the target region is obtained to generate a prediction error signal, followed by transformation coding. The purpose of obtaining a difference with respect to the prediction region is to suppress a difference signal.
In the conventional method described above, a prediction region is obtained by a method like motion compensation after performing padding with reference to the whole image so that insignificant pixels are not included in the prediction region. In the padding process employed in the conventional method, using a pixel value of a significant pixel positioned at the object boundary repeatedly, pixel values of insignificant pixels are replaced with this significant pixel value. When two padding values are provided for a pixel due to padding in both the horizontal direction and the vertical direction, these padding values are averaged to obtain a padding value, and pixel values of insignificant pixels are replaced with this padding value. By padding the whole image in this way, a prediction region of less error can be obtained especially for an image that moves greatly.
However, in order to perform padding while referring to the whole of the reproduced reference image, the whole reference image must be decoded before starting the padding. Further, when the padding is repeated, the amount (complexity) of arithmetic operations (hereinafter, referred to simply as arithmetic amount) increases with an increase in the image size. That is, a delay occurs when the image is reproduced, sometimes leading to an enormous arithmetic amount.
As an arithmetic method in which the arithmetic amount is not proportional to the image size, there is a method of padding reproduced boundary regions region by region. This method can solve the problems relating to delay and arithmetic amount. In this method, however, since only the boundary region is padded, a significant region is restricted to a region within the padded boundary region. Hence, a prediction signal of less error cannot be generated for an image that moves greatly.